To increase corrosion resistance, to enhance the appearance of the surface and for the benefit of any further processing, steel strips are often coated electrolytically with tin, chromium or another metal. The application onto a steel strip of successive layers of different metals is also known.
As a basic material are normally used steel strips which are obtained by the following processes: hot rolling, pickling, cold rolling, cleaning, annealing and temper rolling. For electrolytic application of a coating the following processes are also carried out: de-greasing, pickling, electrolytic coating one or more times, possibly applying a flux material, melting-on of the top coating and post-treatment. Here the fluxing process is intended to create good flow of the last-applied coating during the subsequent melting-on process. All this has the purpose of obtaining a strip of shiny appearance and possibly also to improve the adhesion of the coating material to the steel substrate.
It should be noted that some of the various processes mentioned above may be carried out in combination or omitted without any effect on the essence of the invention.
As a rule melting-on of the outer coating takes place in a vertical melting-on tower through which the strip is vertically fed successively upwards and downwards. Such towers are well known in the art and are described in, for example, W. E. Hoare et. al. "The Technology of Tinplate", 1965, pages 242-243, Edward Arnold (Publishers) Ltd., London. The melting-on takes place by the strip being heated, for example by high frequency electrical induction heating and/or electrical resistance heating, to above the melting point of the outer coating.
With the known methods difficulties can arise with respect to the appearance of the coated strip after melting-on. A particular problem is that of the surface of the strip after melting-on displaying the so-called woodgrain effect (German="Holzmaserung"). This effect is described in W. E. Hoare et. al. "The Technology of Tinplate" (supra), pages 284-288. This effect is known to occur particularly where alternating current resistance heating is used in the melting-on process for heating the strip, but it may also occur when other forms of heating are used, for example high frequency electrical induction heating.
In United Kingdom Patent GB-A-7lO8l5 there is described an apparatus for the melting-on of a metallic coating of a wire or strip, the apparatus comprising a first heater for generating part of the necessary heat for the melting-on by alternating current resistance heating and an additional heater for supplying the remaining necessary heat directly to a small zone of the wire or strip to complete the melting-on in that zone. The woodgrain effect is not mentioned.
Dutch patent application 7612517 proposes that in at least the section of the steel strip in which the temperature of the last-applied coating increases by 20.degree. towards its melting point, an atmosphere is maintained in which the concentration of O.sub.2 is lower than that of the surrounding air by a factor of at least 7. However, it has been found that this measure does not adequately prevent the woodgrain effect in all cases .